Exhaust gas discharged from internal combustion engines such as vehicle engines contains hazardous gases such as carbon monoxide (CO), nitrogen oxides (NOx), and unburned hydrocarbon (HC). An exhaust gas purifying catalyst (i.e., so-called three-way catalyst) capable of decomposing such hazardous gas contains, as a co-catalyst having oxygen storage capacity (OSC), a ceria-zirconia composite oxide or the like. A material having oxygen storage capacity (OSC material) has an effect of suppressing decrease in purification ratio due to variation in the exhaust gas composition by absorbing/releasing oxygen and thereby controlling the air-fuel ratio (A/F) in a micro space.
In order to improve purification performance of an exhaust gas purifying catalyst, an OSC material is required to have a desirable oxygen absorption/release rate to respond to a sudden atmosphere change due to variation in A/F ratio and desirable oxygen storage capacity to maintain oxygen absorption/release capacity for a long period of time. In response to such requirements, for example. Patent Literature 1 suggests an exhaust gas purifying catalyst which exerts a high NOx purifying ability even after a durability test, which comprises a first oxygen storage material on which no noble metal is supported and which has a pyrochlore phase type regular array structure; and a second oxygen storage material which has a higher oxygen storage rate and a lower oxygen storage capacity than the first oxygen storage material, wherein a platinum group noble metal is supported on the second oxygen storage material.
Meanwhile, for example, a catalyst having two catalyst layers (i.e., upper and lower layers) that separately contain Pt and Rh, respectively, has been suggested in view of problematic deterioration of catalyst activity due to solid solution formation of metals in a NOx storage-reduction exhaust purification catalyst that stores NOx in lean atmosphere in which oxygen excess, and releases the stored NOx by changing the exhaust gas atmosphere into stoichiometric or rich atmosphere in which reducible components excess, and purifies NOx by reacting it with reducible components such as HC and CO by the effect of noble metals for reduction (Patent Literature 2 and 3).